Electrolytic recovery of metals



Oct.' 12, 1943.

W. C. HOLMES ELECTROLYTIG RECOVERY OF METALS Filed lom. 1o, 1940 3ra/umml Mae/mex Patented Oct.. 12, 1943 2,331,395 ELECTROLYTIC-REQOVElitYl FMETALS i William Church Holmes, Kellogg, Idaho, asslgnor to SunshineMining Company, Kellogg, Idaho,

a corporation Application October 10, 1940, Serial No. 360,658

8 Claims. (Cl. 2011-105? This invention relates to the recovery'ofmetals and more particularly to the recovery of metals of the groupreferred to in qualitative chemical analysis as the tin group, namely.mercury, ar-

senic, antimony and tin.- The process is yparticularly adapted to therecovery of-,antimony and will be described in detail with reference tothis metal. However, it is to be understood that s the process may alsobe applicable to the recovery of the other named metals. l

Theprocess is particularly applicable to the extraction of these metalsfromv ores containing them in the form oi' sulfides. However, theprocess is equally as applicable to the extraction of these metals fromany materials which contain them in the form of suldes. With referenceto the recovery ofantimony, this process has particular applicability tothe extraction of that It is, therefore, an object yof theI present1nvention to.provide a process for the extraction 'of the tin groupmetals from materials contain-f ing them in the sulfide' form and forthe regeneration of the extracting solution.

It'is another object of my invention to pro- Y vide for the extractionof antimony from argenmetal from argntiierrous tetrahedrite. The lattermaterial is a complexsulfide of silver, copper l and antimony.

To the best of my knowled'gefargentiferrous tntrahedrite has never beenpreviously decomposed for the commercial recovery of antimony without lapreliminary heat treatment involving either roastingor fusion. Due tothe methods of treatment in vogue at ',smelters, for this type oimaterial, it is customary for them to charge never been commerciallyfeasible because, among other things, no one had ever devised a.successful l method for regenerating the sulide solution. Due

to the fact that the sulde solution oxidizes in the leaching andelectrolysis steps, impairing its efficiency, it was necessary to rejecta portion of the so1ution from time to time and replace it with freshsulfide solution and thus the cost became prohibitive.

' In the present invention I have devised a method whereby the abovementioned metals may be ,recovered from their ores by the hydrometallurlterials and products'resulting from the fusion of vtiferroustetrahedrite concentrates by the hydrometallurgical method and for theregeneration ol' the extracting solution.

It is another object of the present invention to provide a process forthe extraction of arsenic and antimony from smelting 'by-products, suchas nue dust, in which the metals are present as oxides and for theregeneration of the extracting solution.

It is another object of the present invention to provide a process forthe extraction of antimony from the product or products obtained fromthe roasting of antimony bearing ores or concentrates,l theantimony-being present in an oxidized form and for the regeneration ofthe extracting solution.

It is another object of the present inventionv to provide a process forthe extraction of antimony contained in the product or productsresulting from the fusion of antimony bearing materials in the presenceof sulfur and for the regeneration of the extracting solution. The mostsuitable of these fused products are known as' matte and Speise.

Itis a further object of my invention to promaterials containingantimony including antimony ores and concentrates in their naturalstate, roasted ores, mattes, speise, products res ulting from the fusionof antimony bearing maantimony bearing materials with other materialsand the regeneration of these leaching solutions.

It is a further object of the present invention to provide a process forthe regeneration of the 'oxidized compounds of an alkaline sulde soluition, such as the sulfate, thiosulfate, sulflte and carbonate salts ofthe alkali metal, back to the sulde condition.

It is a lfurther object of the present invention to provide a cyclicprocess vfor the regeneration o! the oxidized compounds of an alkalinesulde solution. such as the sulfate, .'thiosulfate, sulflte Agicalmethod and whereby the extracting solution may be regenerated in amanner to make the process commercially feasible'.

and carbonate salts .of the alkali metal, bark to the suliide conditionand for the. reduction by a heat treatmentof theYrecovered,precipitatedv salts from this regenerationto obtain a productthat may be used to regenerate `a following batch of oxidized alkalinesulfide solution. A

It is a further object of my invention to provide a continuous method ofcarrying out these extractions and regenerations.

The invention will be described with reference to the recovery ofantimony from argentiferrous tetrahedrite, and in connection with theattached drawing which is a flow sheet illustrating a specificembodiment of the process.

The recovery of antimony from argentlferrous tetrahedrite has beenselected as illustrative of the invention since this presents the4embodiment of greatest industrial importance at present. However, yitsapplicability to the treatment of other materials referred to above isto be understood.

As will be seen from the iiow sheet, the process includes dissolvingantimony in an alkaline sulfide solution, separating the solution fromthe insolubles, precipitating the antimony, and regenerating theleaching `solution by means of a solution of a soluble sulfide of acation whose sulfate, thiosulfate. sulilte, and carbonate aresubstantially insoluble in the lalkaline sulde solution.

More specifically, the process includes leaching the argentiferroustetrahedrite concentrate or other material in nnely divided conditionwith an alkaline sulde solution. The alkaline sulfide solutions can beeither the mono-sulfide or the polysulilde or a mixture of both ofsodium.

potassium or ammonium. With the use of sodium sulfide as the solventthis results in the formation of sodium thio-antimonite, while the useof sodium polysulilde as the solvent results in in the leaching solutionat a minimum as the` presence of thio-antimonate in the pregnantsolution lowers the current emciency of the electroLvtic antimonydeposition. The copper and silverl are substantially insoluble in thesulfide solution and remain behind to be treated by conventional methodsfor the recovery of copper and silver.

'Ihe pregnant solution is ltered or otherwise separated from theinsoluble residue, the latter being washed with wash water from aprevious leach, followed by a washing with water much in theconventional manner of washing illter cakes. The first wash watercontains an appreciable amount of solubles and can be added to thepregnant solution or it maybe recycled as desired.

interposed between two sheets of canvas, the

whole stitched together to give a quilted effect, has been foundsatisfactory. -When new. these `diaphragms are porous and allow somemixing of the anode and cathode electrolytes. This may be obviated byforming a precipitate in the pores of the diaphragm as by soaking it ina solution of soluble sulfate, such as. sodium sulfate, and thereafterimmersing it in a solution of a soluble barium salt, such as, bariumsulfide,V thus precipitating. barium sulfate in the pores of thediaphragm. A single sheet of heavy cotton duck maybe used as thediaphragm or a sheet of microporous rubber would be satisfactory.

A caustic soda solution is used in the anode compartment and is calledthe anolyte. The pregnant solution is usedin the cathode compartment andis called the catholyte. Metallic antimony is precipitated on thecathodes and can be removed therefrom and refined by ordinary thermalmethods,

In the cathode compartment of the cell as electrolysis proceeds sodiumions migrate to the cathode where they-glve up their charge and formmetallic sodium. This metallic sodium immediately reacts with water toform caustic soda and free hydrogen, the latter immediately reactingwith the thio-antimonite and thioantimonate ions, reducing the antimonyto the metallic state and freeing the sulde ions. Some of these sulfideions react with the sodium ions forming sodium sulfide and some migrateto the anode compartment where they give up their charge and enter intothe anodic reaction.

In the anode compartment the hydroxyl ions migrate to the anode,liberating oxygen. This oxygen reacts with the sulfide ions which havemigrated into the anode compartment from the cathode compartmentoxidizing them through the various oxidation stages of 'sulfur finallyforming sodium sulfate. If the caustic soda con centration is sufficientthe reaction will stop at this point. Howeverl if the caustic sodaconcentration drops too low, sulfuric acid will form at the anodesurface which will react with the.

various sodium, sulfur and antimony salts in solution precipitatingelemental sulfur and antimony sulfide and evolving hydrogen sulde.

In the course'of the extraction and the electrolysis the sulde in thecatholyte is,'to a. considerable extent, voxidized to the sulilte, thesulfate and the thio-sulfate condition. In addi tion, a certain amountof sodium carbonate has been formed. These compounds are of no value asasolvent for the antimony in the tetrahedrite concentrates, and,therefore. for a practical proc ess, it is necessary that these productsbe regenerated to the sulfide state.

This forms the most important part of my invention and is accomplishedby allowing the above named salts to build up in the leaching solution,to some extent, but not far enough to impair the eiiiciency of theprocess. A barium sulfide solution is then added under suitableconditions of heat and volume and the two solutions.

The reactionsare as follows:

'I'he resulting sodium sulfide solution is then separated by settlementor filtration or both according to standard methods and the precipitatedbarium salts recovered.

sulfide.

from theelectrolytic cellafter mixing` it with a portion of the fouledanolyte, but I wish it to be understood that the catholyte and theanolyte may be regenerated separately and at any desired point in thecycle without departing from the spirit of the invention.

It is to be noted that all ofthese reactions depend upon the fact thatthe barium salts of the undesirable anions are insoluble, therebymaking' it possible to use barium suliide which has an appreciablesolubility to convert the sodium compounds into suliides and precipitatethe undesirable anions.

I prefer to carry on the regeneration by mixing with the strippedcatholyte a solutlonof barium suliide rather than by the addition ofsolid barium If the solid were used, any insoluble residue therefromwould become mixed with the precipitate and would interfere with thesubsequent regeneration of the barium precipitate. However, while lesssatisfactory, the solid can be used if desired.' l

It has been vround that an excess of barium suliide is undesirable sincethe presence of barium suliide in the regenerated catholyte tends toprecipitate some of the antimony salts. In order to avoid this, I maycarry on the regeneration so Vas to end with a slight deiiciency ofbarium suliide. 'I'hat is, I may avoid this' diiiiculty by not quitecompletely regenerating the catholyte.

The regenerated sulde solution is then returned for use in extractinganother batch oi concentrate. The recovered precipitated barium saltsare either dried or not as is desired and then mixed with somecarbonaceous material, prefer-ably coal, and suiiicient ground barite or'barium sulfate to make. up for yany losses of c are reduced to bariumsulfide, which is in a calcined product known as black' ash. In order tocut down dusting while charging the furnace and during the heattreatment, I may mix the precipitated bari-um salts with av combustibleliquid of high viscosity containing a non-volatile residue. A tar fromthe. destructive distillation oi any carbonaceous material, such as, gastar from the manufacture of articial gas, or still bottoms nde ionsmigrate through the diaphragms into the anolyte and are there oxidized.through various stages eventually to the sulfate condition. Thissulfate ion unites with a sodium ion to form sodium sulfate and as aresult thereof the alkalinity of the anolyte is reduced. This reductionin alkalinity is also partly due to the migration of the sodium ionsfrom the anolyte to the catholyte. Eventually it becomes necessary toadd more caustic soda to maintain the alkalinity of the anolyte.Likewise, the concentration of the sodium sulfate builds up to .such anextent as to increase the voltage drop between the anodes and cathodesand to be generally troublesome by crystallizing out of the solution.When this stage isl reached I may remove a portion of the anolyte andcool it suiliclently to crystallize out a substantial portion of thesodium sulfate after which the anolyte may be returned to the anodecompartment. Instead of doing this, I may remove a portion of theanolyte and add it to the stripped catholyte so that it is absorbed inthe leaching circuit, the sulfate being precipitated as barium sulfateduring the course of the regeneration of the catholyte. l

The decision as to whether the fouled anolyte will be absorbed in theleaching circuit or not is determined by the volume, alkalinity, andsulfur content of the solution in the leaching circuit. There is a lossof sodium suliide in the leaching step and this loss is made up byregenerating part of the spent anolyte. Any excess sulfur that` shouldbuild up in the circuit may be removed by the crystallization of sodiumsulfate from the anolyte. Therefore, part of 11e anolyte will beabsorbed in the leaching circuit and partwill be decrystallized. Myexperience has indicated that the bulk of the fouled anolyte will beabsorbed in the leaching circuit.

Silver sulfide issubstantially insoluble in the alkaline sulfideleaching solution. However, it is more soluble than is the coppersuliide. I have found that antimony recovered as described above fromargentiferrous tetrahedrite contains fromsix to ten ounces of silver perton. I may mony. Other metals might present problems in separation. Thisprecipitation of the silver may or may not be employed, as desired.

Example 3 kilograms of tetrahedriteconcentrate were .leached in (ilitersof a leaching Vsolution containing approximately 125 grams of sulfur perliter, most of which was present in the form of Sodium sulfide. Thesolution had an alkalinity lequivalent to about 250 grams per liter interms of'caustic soda at the start of the leach. The mixture of solutionand concentrate was heated by means of a steam jacket and was agitatedin a steel tank for a period of about three hours. A temperature of C.to 110 C. was maintained for this period.

At the end of this time suiiicient water had 4 evaporated so that themixture almost solidified. When the mixture reached this stage, freshleaching solution w'as added in an amount equal to about to ,120% ofthat previously added. In the instant case,7 liters of leaching solutionwere added at this point. The tank was then covered to cut down onfurther evaporation, and the temperature was maintained within about thesame range. After abouttwo hours, the `ex traction was substantiallycomplete vand the' leaching solution contained approximately 350 gramsper liter of sodium suliide. During the operation thus far,approximately 1.67 liters of solution were evaporated per kilogram ofconcentrate. Y

The mixture was then diluted with wash waters from the filtration of aprevious leach to an alkalinity equivalent to approximately 275 gramsper liter of caustic soda. This dilution improved the filtration step.After dilution the mixture was filtered, and the filter cake washedfirst with a washwater from a previous ltration which wash water wasabsorbed in the pregnant solution until the alkalinity of the latterdropped t approximately 250 grams per liter in terms of caustic soda.After the alkalinity o.' the pregnant solution dropped to the desiredstrength the wash waters were drawn to storage tanks for use in dilutingthe pulp and washing the residue from the following leach. Following theabove cycle, the pregnant solution volume was about 4 liters perkilogram of concentrate leached and contained about 60 grams per literof antimony, most of which was in the form of sodium thio-antimonite andonly a small part as sodium thio-antimonate.

To this pregnant solution sufiicient antimony was added to precipitatethe lsilver which was removed by settlement and filtration. Theclarifled pregnant solution was then conductedl to a special diaphragmelectrolytic cell operated at approximately 50 C. where it was subjectedto electrolysis;

The stripped catholyte was regenerated by means of a hot solution ofbarium sulfide containing between 150-300 grams barium sulde per liter.This operation was carried on at approximately 100 C. .The precipitatedoxidation products were filtered and the cake washed with wash waterfrom a previous operation and with fresh water. The wash water can beused to dilute the regenerated catholyte or can be used as wash water towash the filter cake from the antimony extraction. The filter cake fromthe regeneration was dried and mixed with crushed coal and some groundbarite and reduced to barium sulfide underreducing conditions in afurnace. The black ash resulting from the reduction was leached with hotwater and tlltered to yield a hot barium sulfide solution which can beused for a subsequent regeneration. y

The anolyte at the beginning of the operation was a water solution ofcaustic soda having an alkalinity of about 100 grams per liter ofcaustic soda. After repeated use the anolyte built up a considerableconcentration of sodium sulfate. Part of the anolyte was cooled to about5 to 10 C. whereupon considerable sodium sulfate crystallized from thesolution and was removed. The decrystallized anolyte was then returnedto the anode compartment after the addition of caustic soda or not asthe conditions demanded.

While the above example has treated the individual steps separately, Iwish it understood that the process may be made continuous by providingsuitable apparatus, such as storage bins, agitator tanks, storage tanks,filters, furnaces, electrolytic cells and pumps to contain and circulatethe solution and products, to operate all the various steps in theirproper sequence and at the same time, thus having a continuous cyclicprocess.

While the process has been specifically described with reference to therecovery of antimony from a tetrahedrite concentrate, itis equallyappllcable to the recovery of antimony from other sources, as, forexample, from stibnite, or antimony bearing materials produced byroasting or fusion or as products or by-products of smelting operations.As stated, the invention hasl been found to be peculiarly useful inconnection with the recovery of antimony, and particularly from atetrahedrite concentrate, and, accordingly. the specificationhas beenaddressed specifically to the recovery thereof in accordance with my newprocess. It is nevertheless believed that the invention may have utilitywith varying degrees of success in connection with the recovery ofmercury from such materials as cinnabar. The same is true with respectto the recovery of arsenic which may be recovered from its sulphide oressuch as realgar and arsenical pyrites. Furthermore, the inventionprobably has some utility in the recovery oftin from sources in whichthe tin is present as a sulphide.

As used herein, the term alkali metal is meant to include sodium.potassium and ammonium. While various speciilc embodiments of theinvention have been described, it is to be understood that the inventionis not limited thereto but merely by the appended claims.

I claim:

1. The process of recovering antimony from tetrahedrite concentratewhich comprises contacting said concentrate with a solution of an alkalimetal sulilde, electrolyzing said solution in a diaphragm cell utilizingsaid solution as the catholyte and asolution of a caustic alkali as theanolyte, whereby oxidation products of said sul- 'ilde are formed insaid catholyte land anolyte, regenerating .said sulflde solution bymeans of a barium sulde and adding at least a portion of said anolyte tosaid catholyte before said regeneration, whereby the oxidation productsin said anolyte are regenerated along with the oxidation products insaid catholyte to form regenerated alkali metal sulde.

2. The process of recovering antimony from tetrahedrite concentratewhich comprises contacting said concentrate with a solutionof an alkalimetal suliide, electrolyzing said solution in' a diaphragm cellutilizing said solution as the catholyte and a `solution of a causticalkali as the anolyte, whereby oxidation products of said sulde areformed in said catholyte and anolyte including alkali metal sulfate insaid anolyte. regenerating said sulflde solution by means of a bariumsulfide to form regenerated catholyte and cooling a portion of saidanolyte to crystallize an alkali metal sulfate, removing said crystalsfrom said anolyte and returning lsaid anolyte to said cell.

3. The process of recovering antimony from tetrahedrite concentratewhich comprises contacting said concentrate with a solution of an alkalimetal sulfide, electrolyzing said solution in a diaphragm cell utilizingsaid solution as the catholyte and a solution of a caustic alkali as theanolyte, whereby oxidation products of said suiflde are formed in saidcatholyte and anolyte including alkali metal sulfate in said anolyte,regenerating said sulde solution by means of a barium sulfide, adding aportion of said anolyte tb ."said catholyte before said regeneration,whereby the oxidation products in said anolyte are regenerated alongwith the oxidation products in said catholyte to form regenerated alkalimetal sulfide solution, and cooling another portion of said anolyte tocrystallize an alkali metal sulfate, removing said crystals from saidanolyte and returning said anolyte to said cell.

4. The process of recovering antimony from materials containing thesame, which comprises,

dissolving the antimony in a leaching solution ofan alkali metal sulde.electrol'yzing said solution in a diaphragm cell utilizing said solutionas the catholyte and a solution o'f caustic alkali as the anolyte todeposit said antimony on the cathode of said cell and produce a spentleaching solution Acontaining oxidation products of said sulfide in saidspent solution, regenerating said spent solution with barium sulfide toprecipitate said oxidation products as insoluble compounds'o'f barium Acatholyte and a solution of caustic alkali as the anolyte to depositsaid antimony on the cathode of said cell and produce a spent leachingsolution containing oxidation products of said sulde in said spentsolution, 4regenerating said spent solution with barium sulfide toprecipitate said oxidation products as insoluble compounds of barium andreform alkali metal sulde, separating said precipitate from theresulting solution to are formed in said catholyte and anolyte,regenerating said sulnde solution by means oi barium sulfide and'addingatleast a-portlon oi said ano- Y olyte, whereby oxidation products ofsaid sulfide lyte to said catholyte before said regeneration,

metal sulphide'.` 7; The process of recovering materials containingthesame. whichfcomprises;

` contacting saidv materialsgwith-'a solution of an produce regeneratedleaching solution, mixing said precipitate with a combustiblecarbonaceous liquid ci high viscosity and subjecting the resultingmixture to a temperature sumciently high to reduce said insolublevcompounds of barium to barium suliide and utilizing said last mentionedbarium suliide for again regenerating spent leaching solution.

6. The process of recovering antimony from materials containing thesame, which comprises. contacting said materials with a solution of, analkali metal sulfide, electrolyzing said solution in a diaphragm cellutilizing said solution as a cathn olyte and a solution of causticalkali as the an-V alkali metal sulfide, electrolyzing said .solution4in a dlaphragm'cell utilizingsaid solution as the catholyte and asolution of ycaustic .alkali asthe. anolyte, whereby oxidation productsof said sul-* ilde are formed in said catholyte and anolyte includingalkali metal sulfate in said anolyte, regenerating saidv sulfidesolution by means of a barium s ulde'to form regenerated alkali metalsuliide solution, cooling a portion of said anolyte to crystalllze analkali metal sulfate, removing v'antimoriy from whereby the oxidationproducts 'in said anolyte'V are regenerated along with the oxidation-products in said catholyteto form regenerated alkali` said crystalsfrom said portion of said anolyte and returning said' portion to saidcell.

8. The process of recovering antimony from materials containing thesame, which comprises, contacting said materials with a solution of analkali metal sulfide, electrolyzing said solution in a, diaphragm cellutilizing said solution as the catholyte and a solution of causticalkali as the anolyte. whereby oxidation products of said sul- Aiidearei'ormed in saidcatholyte and anolyte including alkali metal sulfatein said anolyte, re-

generating said suliide solution by means of al barium sulfide, adding aportion of said anolyte to said catholyte before said regeneration,whereby the oxidation products in said analyte are regenerated alongwith the oxidation products in said catholyte to form regenerated alkalimetal suliide solution, and cooling another portion of said anolyte tocrystallize an alkali metal sulfate,

removing said crystals from said` other portion of anolyte and returningsaid other portion of said anolyte to said cell.

WILHAM CHURCH Homme.V

